1. Field of the Invention
The present invention relates to an optical switch having waveguides. More particularly, the present invention pertains to a semiconductor optical switch which is suitable for use in an optical integrated circuit or opto-electronic integrated circuit employed for optical computing, optical information transmission, etc.
2. Description of the Related Art
The inventors of this application have already made some proposals concerning the structure of semiconductor optical switches, i.e., a semiconductor waveguide type optical switch which has a short device length and therefore enables large scale integration on a semiconductor substrate, a slip structure type optical switch and an integrated semiconductor, waveguide type optical switch which is employed for an photonic switching. We discussed them, for example in IEEE Journal on Selected Areas in Communications, J-SAC-6, pp. 1262-1266, 1988.
The structure of the optical switch array and that of an optical switch constituting the array, which have heretofore been proposed by the present inventors, will be explained with reference to FIGS. 2A, 2B and 2C. A 4.times.4 optical switch array is shown in FIG. 2A. A plurality of optical waveguides are provided on a semiconductor substrate. The plurality of waveguides have a plurality of X-crossing waveguides and are optically coupled to optical fibers 201 on the signal input side and optical fibers 202 on the signal output side. An enlarged view of a unit cell of an optical switch that constitues an X-crossing waveguide is shown in FIG. 2B. An additional waveguide is connected to respective parts of waveguides that cross each other in the shape of an X, thus defining two Y-branches 203. These waveguides 206 are produced in such a manner that a semiconductor material, for example, inGaAsP, is grown on an InP substrate 204 to provide a layer 205, which is then subjected to, for example, reactive ion-beam etching using Cl.sub.2 or the like with a photoresist used as a mask, thereby forming a ridge-shaped structure. The sectional area of the ridge waveguides 206 is about 5 .mu.m width.times.1.5 .mu.m height. The sectional view of the optical switch shown in FIG. 2B, taken along the line A-A' is shown in FIG. 2C on the InP substrate 204 are successively grown a waveguide layer 205 (bandgap wavelength: 1.15 .mu.m) of n-InGaAsP, a cladding layer 206 of n-InP and a cap layer 207 of n-inGaAsP by ordinary LPE (Liquid Phase Epitaxy) method. After a carrier injection region 209 has been formed by diffusion of Zn or the like, an insulator layer 208 of SiO.sub.2 or the like and a p-side electrode 210 are formed thereon by evaporation or other similar means. An n-side electrode 211 is similarly provided on the reverse side of the InP substrate 204. This optical switch, which utilizes a refractive index change on the basis of carrier injection, is relatively large in the amount of change in the refractive index of the constituent material and therefore suitable, particularly, for fabricating an optical switch array having small-sized optical switches and a large number of light input and output waveguides.
However, it was found as a result of further studies made by the present inventors that the above-described optical switch or optical switch array involves some propagation losses, for example, increases in the loss and crosstalk caused by scattering of light at the branches and inferior optical switch characteristics, and waveguide loss intrinsic to the waveguides. These propagation losses are technical problems which cannot be ignored when it is intended to integrate optical switches in a large size and at a high density.